Composite materials, e.g. comprising an array of fibres impregnated with a cured resin, are widely used in the manufacture of light weight structures. However, such structures suffer from being vulnerable to electromagnetic hazards such as lightning strikes, causing damage to the structure, which is a particular problem for aircraft structures. Varying techniques and methods have been suggested in the prior art to provide lightning strike protection to composite materials, all involving the addition of conductive elements.
U.S. Pat. No. 4,429,341 discloses a method by which a graphite epoxy composite is layered with one or more layers of dielectric material applied over the exposed composite surface portion of an aircraft, the outermost layer of dielectric material (Kapton®—polyimide) having a binder on the exposed surface thereof for holding a finely divided (conductive) metal powder (e.g. aluminium) distributed uniformly over the surface of the outermost layer of dielectric material.
EP 0318839 discloses a method by which a wire grid (titanium, diameter 1.27 mm) is disposed intermediate two structural panels of a fibrous graphite material. A further conductive strip (titanium) is positioned around the periphery of the lightning protective skin member and electrically coupled to the wire members of the grid such that all the wire members are electrically coupled together. Titanium is the preferred metal as it prevents corrosion problems encountered when using other metals in contact with graphite.
WO 2004/033293 discloses a method to protect aircraft from lightning strikes by fabricating cheaper aircraft panel assemblies comprising a honeycomb core surrounded by a plurality of inner filler ply layers and a plurality of inner prepreg layers. At least one metal foil layer, more specifically an aluminium foil layer, is placed on the outer layer of the plurality of inner filler ply layers and inner prepreg layers.
US 2006/0078705 discloses a method for repairing fibre-reinforced composite structures while maintaining original EM and lightning protection using carbon nanotubes, fibres and thermoset resins. The electrical conductivity of the carbon fiber composite material is further modified with additional components, preferably carbon nanotubes, and more preferably carbon nanotubes replete with additional carbon-based materials (e.g. Buckyballs, fullerenes, carbon black), and other electrically conductive materials such as organic and inorganic metal compounds (e.g. indium tin oxides and zinc oxides).
EP 0629549 discloses an upper layer composition for “strengthening” an epoxy composite structure comprising a copper mesh. The upper layer comprises nickel-coated carbon (or Kevlar® aramid) fibres woven into a cloth. The coefficients of thermal expansion for copper (19 ppm/° C.) and the epoxy composite (1 ppm/° C.) are sufficiently different such that if not properly bonded, this difference causes micro-cracking of the epoxy surface, and corrosion of the copper mesh ensues. Layering the composite with an additional upper layer of woven electrically conductive nickel-coated carbon (or Kevlar® aramid) fibres, provides a suitably strengthened composite with enhanced lightning strike protection, with only a minor increase in weight (200 g/m2).
US 2004/0084103 discloses a composite preform structural panel comprising electrically conductive (metallic thread) stitching, whereby the stitching forms an electrically conductive grid-like network to better dissipate electrical energy received in a lightning strike. Disclosed in the embodiments is a metallic thread with a preferred diameter in the range of 1.5 to 750 μm, more preferably 250 to 750 μm, most preferably 400 to 700 μm. Preferred metals include stainless steels, Nickel 200, Copper 11000, Titanium (CP), brass, Hastalloy X, Hastalloy C-22, NiChrome and aluminium.
EP 0913498 discloses a process of depositing metal onto solid polymer surfaces, e.g. printed circuit boards.
US 2002/0154427 discloses a process of applying a metallic mirror surface to solid substrates.